This is a really hard one to answer, let me tell you. I have warred with myself repeatedly on what advice to give you. I read this really great article last week Traditional Publishing and it's Suppliers where it talks about how the publishing industry treats authors like vendors and that the publishing industry makes decisions based on what is best for their business, not on what is best for the author. Then I read another article, which sadly I forgot to bookmark, but it talks about how writers do not treat their work as a business and because of this, independent writers as a whole will fail.
That being said, I decided to give you the advice I followed and charge you to make the decision for yourself and what you think is best for your business. Now, most traditional publishing agents out there insist that you should query one agent at a time, wait a minimum of six weeks or until you get a rejection letter, and then send your query to the next agent. It is a "professional courtesy" or some such nonsense. So, if you can wait three years to get an agent, and another one or two years for your agent to get a publisher and then another one or two years to get your book published, by all means, follow their advice. (For those of you doing the math, that is five to seven years to get published.)
But if you want writing to be your business, if you need to make money on your investment, if you cannot put your dream on hold for the next five years then I say: submit to multiple agents at a time. Just don't submit to multiple agents in the same house at the same time. Agents talk. Whether or not you let the agent know that you are submitting to other agencies is up to you. Some people think that it will get them a more timely response, but many agents have intimated that it will simply get your letter tossed.
Now, I know some of you are thinking "why should we listen to her? She is a self-pubber." That is cool. Here is an example: My first book All is Well, I shopped around for two years following the traditional wait-until-you-get-rejected philosophy. After two years, I started mass submissions because I was, quite frankly, getting desperate. A year of that and I finally got a contract offer. I was so excited that I didn't do a lot of research on the agency to which I had submitted. I took a cursory glance and saw that they had books published and signed my book away for another year. In that year, I got three publisher contracts from that agent. All were new companies that basically would provide me the same services that I could achieve myself as an independent publisher, so I passed. After my year contract was up, I did not bother trying to get it renewed and neither did my agent. I could have self-published a couple of years ago and be rolling in the dough. (At least, I would like to think!) And you are thinking "Well, maybe her book wasn't that good."
Okay, it is possible,. I am personally invested and therefore biased, despite the fact that everyone who has read it has praised it. So, here is another example: My uncle, Mike Dunbar is a world-renowned writer and maker of Windsor chairs. His first book was published in 1976 and he has had six other books published, all on chair making. He has also written multiple magazine articles on chair making, has been a contributing editor for three magazines and a columnist for two. In other words, the guy is a good writer.
About four years ago he decided to follow in Rick Riordan's footsteps and write a mid-grade series with a sci-fi twist. As soon as he finished the first book, he began submitting and using his agency contacts to find someone to pick the series up. He just finished book seven and has had a couple of agents who played with being interested, but never followed through. He then shopped it around one at a time, per industry standards, still trying to work contacts. In the end he was submitting it to anyone he could find as fast as he could send the submissions out.
Now, perhaps I am biased, but I have read them all and they are good! He has had them handed around at local schools and I have even shared it with my own cub scouts and every kid who has read one of the books has loved it! But he still hasn't gotten a contract. Why? Part of it, I suspect, has to do with the way the industry is going. If Mike wrote a book on wood crafting, it would get picked up in a heartbeat. The publishers know those books sell well. But they are afraid to take a risk on his new path. Mike is old-school and is only just coming around to the idea of self-publishing his books. If he is thinking about it, why shouldn't you?
But the point of this article is not to convince you to self-publish. It is simply for you to be well-informed before you go off querying, so that you can make the best decisions for your business as possible. So here are some more statistics you should be aware of:
"A Wrinkle in Time" by Madeleine L'Engle was rejected 29 times.
"Auntie Mame" by Patrick Dennis was rejected 15 times before it went on to be a major hit.
"Chicken Soup for the Soul" by Jack Canfield and Mark Victor Hansen was rejected 140 times.
"Zen and the art of Motorcycle Maintenance" by Robert Pirsig was rejected 121 times.
"Carrie" by Stephen king was rejected 30 times.
C.S Lewis was rejected over 800 times before selling his first story.
Louis Lamour was rejected over 200 times before selling his first story.
Agatha Christie waited four years to finally publish her first book.
"A Time to Kill" by John Grisham was rejected by sixteen publishers before an agent picked him up and then dropped it as well.
J.K Rowling submitted Harry Potter to 12 different publishing houses, who all rejected the work.
"Twilight" by Stephenie Myer was rejected 14 times before an agent landed on it.
So, what is the point? The point is, even big authors got rejected. Google "authors who have been rejected" and you will see some CRAZY stuff! Agents aren't psychic. They judge books on their interests and some research on the way the industry is currently flowing. The industry is constantly changing. They will all have different opinions. The question you have to ask yourself, is how long are you going to wait on their fickle opinion? Again, only you can make this call. It is your book, your business, your style.
I hope this was helpful and informative. If you have any questions about queries, feel free to ask in the comments section below!